A typical truck loading dock of a building includes an exterior doorway with an elevated platform for loading and unloading vehicles such as trucks and trailers. Many loading docks have a dock leveler to compensate for a height difference that may exist between the loading dock platform and an adjacent bed of a truck or trailer. A typical dock leveler includes a deck that is vertically movable for adjusting the height of its front edge to an elevation that generally matches the height of the rear edge of the truck bed. A pivotal or otherwise movable lip extending from the front edge of the deck can be placed upon the truck bed to form a bridge between the deck and the bed. This allows personnel and material handling equipment, such as a forklift truck, to readily move on and off the vehicle during loading and unloading operations.
To help prevent the truck from accidentally pulling away from the elevated platform of the dock while the truck is being loaded or unloaded of its cargo, the loading dock may include a vehicle restraint. A vehicle restraint usually includes a hook or some other type of barrier that can move to selectively engage and release some part of the truck or trailer, such as its wheel or the rear impact guard (ICC bar).
Many dock levelers and vehicle restraints are actuated by a hydraulic system comprising various configurations of cylinders, rotary actuators, pumps, valves and other hydraulic components. Although hydraulic systems are excellent means for actuation, the hydraulic fluid used in such systems is usually petroleum based and not biodegradable, thus the fluid may create an environmental problem. If a non-biodegradable fluid, for instance, were to leak out onto the dock floor, the fluid may drain into a storm-sewer meant primarily for handling relatively clean runoff. But even if the fluid does not accidentally escape, hydraulic fluid should be changed periodically to prolong the life of the equipment, and procedures for properly disposing of non-biodegradable fluid can be expensive and/or difficult to expedite.
Although food-grade biodegradable fluids are available for food-handling equipment, such fluids are inadequate for use in loading dock equipment, which operate under a unique set of conditions. First, loading dock equipment is typically outdoors so it may need to operate over a very broad range of temperatures from below −40° F. to over 100° F., which can cause an unacceptable variation in the fluid's viscosity. Common food-grade biodegradable fluids may gel or solidify near 0° F., which is unacceptable for many loading dock applications. At low temperatures, excessive viscosity can slow the operation of a hydraulic system and inhibit the pump's ability to pump the fluid. At higher temperatures, insufficient viscosity can allow surface wear to occur between moving parts, i.e. pump gears, piston seals, etc.
Second, dock levelers and vehicle restraints have such a low duty cycle that in some cases they may only operate a few times a day with an operating duration of just 30 seconds per cycle. The short cycle times and extended periods of inactivity make it difficult for the operation of the pump itself to keep the hydraulic fluid sufficiently warm for proper viscosity.
Third, since hydraulic systems of loading docks are usually exposed to outdoor air, they are particularly subject to moisture contamination. Hydraulic systems typically have a tank or oil reservoir for holding a supply of hydraulic fluid from which the pump draws the fluid. As hydraulic cylinders extend or retract to move the dock leveler or vehicle restraint, oil leaves or returns to the tank, which causes the fluid level in the tank to rise and fall. The varying fluid level forces ambient air to alternately enter and leave the tank through what is known as a breather cap. Outside air entering the tank can introduce moisture to the hydraulic fluid. If the outside air temperature cools the tank, warmer air inside the tank can release moisture that condenses on the inner surface of the tank and eventually drains into the hydraulic fluid. Moisture not only promotes the degradation of biodegradable fluids, but it also dilutes and shortens the useful life of non-biodegradable fluids as well.
Consequently a need exists for a loading dock system that can operate with a biodegradable hydraulic fluid.